1. Field of the Invention
The invention relates to a process for manufacture of flat composites of plastic and to the resulting plastic composites.
2. Discussion of the Background
European Patent Application EP 584593 describes plastic composites in which at least one part of the composite comprises acrylic glass and at least one acrylic-glass joint is formed by welding. Joining by thermal welding is achieved in the presence of an intermediate layer having a thickness of 0.01 to several mm and comprising polymers formed substantially from methyl methacrylate. The shaped parts of acrylic glass can be joined in the presence of the thin intermediate layers of polymethyl methacrylate with shaped parts made of plastics, such as PVC, cellulose ethers, polystyrene, ABS plastics and polycarbonate, especially polycarbonate of bisphenol A.
If the prescribed temperature program is not exactly followed, inclusions of air or monomeric decomposition products occasionally occur during this process in the composite.
European Patent Application EP 710549 describes a process for the manufacture of flat composites from polymethyl methacrylate plastic by thermal adhesive bonding of at least two parts of polymethyl methacrylate plastic heated at their surfaces. The parts of polymethyl methacrylate plastic, which are preheated on the adhesive side to surface temperatures of 130 to 200xc2x0 C. while retaining their original shape during the preheating phase, are pressed together between a pair of rolls with a feed velocity of between 5 and 200 mm/s and with a pressing force of between 10 and 500 N. At least one roll, with which the pressing force is applied, is mounted movably. European Patent Application EP 710548 describes the same process for thermoplastic compositions in general.
Although these processes have been proven in principle, problems occur occasionally when the prescribed temperature program is not exactly followed. Optical distortions, warping and adhesion problems can then occur. In particular, the handling of parts with small thickness has proved to be very difficult. The handling of parts of very different thicknesses with correspondingly different heat-absorption capacities also frequently causes problems in practice.
It was an object of the present invention to develop a process for the manufacture of flat composites of plastic by thermal adhesive bonding that results in composites that do not have the above mentioned problems and wherein the process and the properties of the resulting flat composites are less dependent on temperature fluctuations. In particular, it was an object to develop a process for the manufacture of flat composites of plastic by thermal adhesive bonding wherein the thermal joining of thin parts and parts of different thicknesses, asymmetric joining, with very different heat-absorption capacity is possible. In addition, it was an object to obtain plastic composites having unimpaired strength.
These and other objects have been achieved according to the present invention, the first embodiment of which includes a process for manufacture of flat composites of plastic, comprising:
thermal adhesive bonding of at least two plastic parts which each independently comprise a base material coated with a plastic layer;
wherein each of said plastic parts is heated on their surfaces coated with said plastic layer, thereby providing heated parts;
wherein said heated parts are welded under pressure;
wherein a Vicat softening point of said plastic layer is 5 to 40xc2x0 C. lower than that of said base material; and
wherein said base material has a thickness of 1 to 200 mm and said plastic layer has a thickness of 5 to 400 xcexcm.
In another embodiment the present invention includes a plastic composite obtained according to the above process.